Many devices having audio outputs and/or inputs, such as mobile phones, tablets, computers and the like, are provided with a socket, allowing an audio accessory device with a suitable plug, such as a headset, to be connected to the device.
FIG. 1 illustrates an audio system, in which an audio accessory 20, specifically a headset, is connected to a host device by means of a jack plug 25. The host device comprises circuitry 11, which connects to a number of contacts 26 in a jack socket. FIG. 1 shows the situation in which the jack plug 25 is inserted into the jack socket, so that poles on the jack plug 25 make electrical contact with the contacts 26.
FIG. 1 shows left and right driver amplifiers 102 and 104 driven from respective signal sources VinL and VinR. The signal sources VinL and VinR may for example comprise digital-to-analog converters (DACs) driven by received digital audio signals. These driver amplifiers 102, 104 are coupled to respective jack poles L and R by respective jack socket contacts. Signals from the driver amplifiers 102, 104 are then passed to left and right speakers respectively in the headset 20.
FIG. 1 also shows an amplifier 106 serving as a microphone pre-amplifier coupled to a microphone pole, i.e. pole M, of the jack plug via a respective contact. The amplifier 106 thus receives a signal from a microphone in the headset 20.
The pole G of the jack plug 25 provides a common ground return path for the signal paths through the other three poles L, R & M.
In this example, the plug and socket arrangement is illustrated as being a 3.5 mm 4-Pole TRRS (Tip/Ring/Ring/Sleeve) jack plug and jack socket arrangement. The small size of such a socket allows only a small area of physical, and hence electrical, contact between the jack contact within the socket and the cylindrical pole of the inserted plug. Thus there may be an appreciable contact resistance between one or more respective jack contacts and respective poles, which may be further degraded due to corrosion or foreign matter.
FIG. 1 also illustrates that there may be various parasitic resistances, including a jack contact resistance, in the ground path between the ground jack pole G, i.e. node X4, and a ground reference point X3 close to the circuitry 11. These various parasitic resistances are illustrated as lumped together into a single resistance element RG. There may also be similar parasitic resistances between the microphone pole M of the jack plug and the microphone pre-amplifier 106 via the respective jack socket contact, and FIG. 1 shows these parasitic resistances lumped similarly into a single resistance element RM.
The input signals VinL and VinR of the driver amplifier circuitry 102, 104 and the resulting buffered outputs VoutL and VoutR may be referenced to some ground point X1 local to the driver amplifier circuitry 102, 104. The microphone pre-amplifier 106 may be explicitly or implicitly referenced to a ground point X2 local to the pre-amplifier 106. By careful design of local ground planes or ground tracks on PCBs and/or in integrated circuits, these grounds may be maintained close in voltage to a common ground reference point, say the ground pin of an integrated circuit implementation of the circuitry 11, which in turn may be closely coupled to some local ground reference point X3. However, the voltage signals VoutL and VoutR applied to the left and right speakers may give rise to corresponding ground return currents passing though the jack contact resistance and PCB trace resistance illustrated by RG, so it is likely that the ground voltage on the pole of the jack, node X4, will be significantly different from that at X3 and will also be modulated by a.c. signal components of these ground return currents corresponding to the a.c. signal components of the applied voltages VoutL and VoutR.
FIG. 2 is a more detailed electrical circuit diagram showing the system of FIG. 1.
Thus, FIG. 2 shows the left speaker 108, the right speaker 110, and the microphone 112 of the headset 20.
The current IG through the ground pole G of the jack plug comprises both the currents ISL and ISR through the left and right speakers 108, 110. Thus, ignoring any ground return current corresponding to current though the microphone pole, i.e. M pole, of the plug:IG=ISL+ISR.
Given that the resistance RG is non-negligible, the ground voltage on the ground pole, i.e. G pole, of the jack, at node X4, will differ from that at X3 by a voltage VRG, where:VRG=IG*RG.
The input voltage VmicA to the microphone amplifier 106 will not be identical to the voltage Vmic generated by the microphone 112, but will be modulated by the voltage VRG as it varies with the variation of the total current IG flowing through the left and right speakers 108, 110 into the ground return path. Thus there will be crosstalk of the electrical speaker signal into the sensitive microphone channel (which is especially relevant because a typical microphone signal might be only of the order of 10 mV). That is:VmicB=VmicA=Vmic+VRG.
Also the respective voltages actually appearing across each respective speaker will be reduced by VRG. That is:VSL=VoutL−VRG andVSR=VoutR−VRG 
Since VRG is dependent on both ISR and ISL, one effect is that the respective currents through each respective speaker influence the voltage that is generated across the other speaker, and so there is crosstalk from the right channel to the left channel, and from the left channel to the right channel, which can distort or blur the stereo image.
The ground return path via resistance RG will also carry ground return current from any current taken though microphone pole M of the jack plug, and any modulation of this current may appear as audible crosstalk in the speaker outputs. The current through microphone pole M may comprise supply current for the microphone 112 which may vary with the microphone signal and possibly also if the microphone 112 is turned on or off or passes though different microphone operating modes, possibly autonomously due to some internal voice activity detect or suchlike.
Also in some applications current on this microphone supply line is used to signal to the host device, for example via one or more pushbutton switches coupled between the microphone pole M and ground pole G either directly or via resistances, illustrated by resistance Rpb 113. Use of the one or more pushbutton switches may give rise to significant step changes in the current through poles M and G, which may in turn give audible artefacts through modulating VRG.